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Seat belts 18
Why wear seat belts?
Physical principles of frontal collisions
In the event of a frontal collision, a large amount of kinetic
energy must be absorbed.
It is easy to explain how the laws of physics work in the case of a head-on
collision: When a vehicle starts moving
⇒
fig. 6 there is a certain amount of
energy known as “kinetic energy”, both in the vehicle and in the occupants.
The amount of “kinetic energy” depends on the speed of the vehicle and the
weight of the vehicle and its passengers. The higher the speed and the
greater the weight, the more energy there is to be “released” in an accident.
The most significant factor, however, is the speed of the vehicle. If the speed
doubles from 25 km/h to 50 km/h, for example, the kinetic energy is multi-
plied by four.
Given that the passengers in our example are not restrained by seat belts, the
entire amount of kinetic energy has to be absorbed at the point of impact
⇒
fig. 7 .
Even at speeds of 30 km/h to 50 km/h, the forces acting on bodies in a colli-
sion can easily exceed one tonne (1000 kg). At greater speed these forces are
even higher.
Passengers not wearing seat belts are not “attached” to the vehicle. In a
frontal collision, they will move forward at the same speed their vehicle was
Fig. 6 Vehicle about to hit
a wall: the occupants are
not wearing seat belts
Fig. 7 The vehicle hits
the wall: the occupants
are not wearing seat belts
Ibiza SC_EN.book Seite 18 Donnerstag, 2. April 2009 1:55 13